Direct current (DC) power supplies and DC/DC converters are often connected to have the outputs operate in parallel. This can occur in a case where a system is being upgraded, e.g. to supplement the needs of an existing power supply, or in an initial design environment which allows a purchaser of a system, such as a computer system, to custom configure the system capacity in terms of processor count. For example, the latter situation is common for new parallel processor systems, where customers often have the ability to choose within the same product line anywhere from one processor to hundreds of processors. Rather than redesign the power supply for each variation, the preference is to parallel the outputs of the plural power supplies as the processor count dictates.
Given that the voltages of the parallel connected power supplies will not match perfectly, there is a need for output current sensing and related control to ensure that the current outputs are balances among the power supplies. Output current sensing is also required as a part of the current limiting protection integral to such power supplies. Thus, whether one or multiple parallel connected power supplies are used, load or output current sensing is a necessary aspect of contemporary power supply designs.
The type of circuit utilized for sensing the DC output current from power supplies is an important aspect of the cost and power dissipation. The importance increases as the number of power supplies and output current grow. Also, the trend toward lower power dissipation hardware as motivated by environmental issues also drives the need for lower power dissipation designs.
Current sensing in a power supplies can be achieved in many different ways. In power supplies using transformers with both primary and secondary sides, current sensing can be achieved on the primary side using either a sensing resistor or a current transformer. However, sensing of a secondary load current by measurements performed on the primary side introduces inaccuracies due to the presence of magnetizing current within the transformer. Sensing of the power going to the primary side of the transformer using a resistor does, however, reduce the power dissipation. The current transformer approach suffers not only from the magnetizing current error, but from the relatively high cost of the components for this configuration. Therefore, the preference is to sense output or load current flow on the secondary or load side of transformer configured power supplies.
The sensing and related control of the load current on the output side of the power supply is conventionally done using either a Hall effect device or with a shunt resistor in path of the output current. The Hall effect device tends to be relatively expensive. Though shunt resistors can be expensive if tight tolerances are required, the prevailing problem with such devices is the relatively high power dissipation associated with their presence in the DC load current path.
Therefore, there exists a need for a system and method which provides an accurate measure of power supply output or load current as measured on the secondary side which is relatively low in cost, and which has minimal power dissipation.